During manufacturing, transport, storage, and use of a multilayer photographic film in a roll form, the front (i.e., the imaging side) of the photographic film comes into contact with the film backing. Depending on the severity of this contact, it can lead to ferrotyping, sticking and materials transfer. Ferrotyping refers to the imprinting of a glossy surface onto the front side of a photographic film following intimate contact with the backing. Depending on the composition of the backing, materials transfer that occurs during this intimate contact may have deleterious effects on the sensitometric behavior of the photographic film. To help prevent intimate contact between the front and back sides of a multilayer photographic film, the protective overcoat layer that overlies the imaging layer typically employs matte beads as spacers. These matte beads are hard, inorganic or organic particles such as silica particles or high Tg polymeric beads.
Front-to-back contact may still occur for a multilayer photographic product that contains matte beads in its outermost protective layer, especially for gelatin-based photographic layers exposed to high humidity. We have found that this failure of the matte beads to prevent front-to-back contact arises as a result of penetration of the matte beads into the photographic layers under pressure, for example, when the film is tightly wound into a roll. Although photographic layers may be harder than a matte bead, these photographic layer become softer with time due to their viscoelastic nature. The softening process is further accelerated at higher humidities by the plasticizing effects of moisture. Because of this matte bead penetration, the surface roughness of the photographic product is reduced resulting in the appearance of ferrotyping. Matte penetration can further lead to the dislocation of the silver grains for silver halide-containing photographic layers by contact stresses. The dislocation of the silver grains causes a pressure marking of the photographic product.
Up to now, the mechanisms that cause and the problems associated with matte bead penetration into imaging layers as a result of front-to-back contact have not been fully understood. In addition, the prior art does not discuss methods to prevent such matte bead penetration into photographic layers. To avoid pressure sensitization, the prior art describes adding a soft cushion layer as a stress absorbing intermediate layer between the protective overcoat and photographic layers as disclosed in U.S. Pat. Nos. 5,066,572, 5,300,417, and 5,310,639. However, these soft cushion layers can not reduce matte bead penetration. In fact, such soft layers actually promote matte bead penetration.
U.S. Pat. No. 4,499,179 discloses using a two-layer protective overcoat for a photographic layer in an attempt to reduce pressure marking. The two-layer protective overcoat comprises an outer layer and an inner layer, wherein the ratio of the thickness of the inner layer to the outer layer is at least 1.5. The outer layer contains oil particles in the form of finely dispersed, water-insoluble droplets. The inner layer contains fine particles of an inorganic oxide or polymeric material, and optionally, oil particles analogous to those contained in the outer layer. Such a thick, inner protective layer containing hydrophobic fillers is undesirable since it may retard the image development process and may reduce image sharpness. In addition, since the inner protective layer may contain both oil droplets and fine particles of organic or inorganic material, such a layer may be quite soft due to the presence of the oil droplets and therefore be undesirable as a method to prevent matte bead penetration during front-to-back contact.
The prior art also describes in U.S. Pat. No. 4,822,727 the use of one or more overcoat layers containing polymer latexes having a Tg above 20.degree. C. and polymer latexes having a Tg below 20.degree. C. Such overcoat layers reportedly have reduced brittleness and reticulation while improving sticking resistance. However, by incorporating both the soft and hard latexes in the overcoat layers, the stiffness of these layers may be too low to prevent matte bead penetration during front-to-back contact.
The aforementioned prior art references relate to some aspects of the present invention, but, do not fully consider the problem of matte bead penetration into imaging layers during front-to-back contact, nor do they disclose or suggest an adequate solution to this problem. Therefore, there is a need for an imaging element having an improved interlayer that prevents matte bead penetration and the associated problems of pressure sensitization, ferrotyping, sticking, and materials transfer without compromising image development and image quality.